As described in the above referenced application, in the past, numerous difficulties have been encountered in operating systems employing conveyorized dollies. A specific example of a conveyorized dolly system is a car wash, in which difficulties have arisen as a result of inadequacies in the dollies used for moving an automobile through the car wash.
In a car wash the automobile is driven to the entrance of the car wash. One of the front tires is aligned with a car wash track. The car is then moved forward with the front tire sitting in or on the track. The transmission of the automobile is placed in neutral out of gear and a dolly secured to a pair of drive chains is moved to a position where it engages the rear of the tire. The dolly which is secured to the chains then pushes on the tire to move the automobile through the car wash.
The dolly is supported by the track, and therefore, must include a roller in contact with the track for rolling the dolly along the track as the chains are moved. Since the dolly is moving in the same direction as the automobile, the roller in contact with the track and the automobile tire must rotate in the same direction. If the roller in contact with the track engages the automobile tire, it binds with the tire and rides under the tire because of its direction of rotation. Therefore, the dolly must also include a second roller which is out of contact with the track and which is freely rotatable. It is the second roller which is freely rotatable that engages the automobile tire. The second roller rotates in the opposite direction to both the contact roller and the tire when engaged by the tire. Therefore, the purpose of the second roller is to permit rotation of the tire in a manner such that there is no binding between the dolly and the tire.
As can be appreciated from the above description, the lower reach of the second roller must be above the lower reach of the support roller to prevent contact between the second roller and the car wash track. However, as a result of this positioning of the two rollers with respect to one another, a gap exists between the freely rotatable roller and the track, and in order to compensate for tipping of the dolly, this gap is generally great enough to permit an operator's foot or hand to ride beneath the dolly when it is not blocked by the automobile tire. This therefore necessitates the use of a safety bar for blocking this gap. Such an arrangement is shown in U.S. Pat. No. 3,260,219.
It has been discovered through the use of dollies in car washes that the drive chains cannot be tightened to the extent that tipping of the dolly under the initial load is prohibited. This tipping results inbinding between the safety bar and the supporting track surface, which binding causes damage to both the track and the safety bar, and necessitates the frequent replacement of the safety bar.
The double dolly according to the present invention has been constructed to overcome the above-mentioned drawbacks of the prior art devices. It consists of a first roller which rotates about a first axle, a second roller which is located forwardly of the first roller with its lower reach above the lower reach of the first roller and which rotates about a second axle, a frame for securing the rollers and axles to the conveyor, and a safety bar secured to the frame in advance of and below the lower reach of the second roller. The second roller is freely rotatable and free of load and is positioned to prevent contact between the first roller and the object which the dolly is moving over the supporting surface. The first roller is a support roller for transferring load exerted on the dolly to the supporting surface. The safety bar is clear of the supporting surface when the dolly is free of load and is adapted to rotatably engage the supporting surface and to prevent binding upon coming into contact therewith when the dolly is under load.